Method of forming gallium arsenide films by vacuum evaporation deposition

ABSTRACT

A method utilizing vacuum evaporation techniques for growing monocrystalline films of III-V compounds such as gallium arsenide on structurally dissimilar crystalline substrates. In a preferred embodiment, a single gallium arsenide source is heated to 900*1000*C at a subatmospheric pressure of about 10 5 to 10 8 torr to evaporate gallium and arsenic therefrom for recombination on a smooth, clean single crystal sapphire ( Alpha -alumina) substrate maintained at about 580*-595*C.

Unite States atent Kenty Feb. 4, 1975 METHOD OF FORMING GALLIUM ARSENIDEFILMS BY VACUUM EVAPORATION DEPOSITION [75] Inventor: Joseph L. Kenty,Placentia, Calif.

[73] Assignee: Rockwell International Corporation, El Sequndo, Calif.

[22] Filed: Feb. 4, 1975 [21] Appl. No.: 379,521

[52] U.S. Cl 117/213, 117/106 A, 117/201 [51] Int. Cl. C23c 13/04 [58]Field of Search 117/106 A, 107,201,213; 148/175 [56] References CitedUNITED STATES PATENTS 3,607,135 9/1971 Gereth et al 117/106 A X3,674,552 7/1972 Heywane l17/106 A X OTHER PUBLlCATlONS Powell et al,Vapor Deposition, Joan Wiley & Sons,

New York, 1966, p. 632. Manasevit, Single-Crystal Gallium Arsenide 0nlnsulating Substrates, Applied Physics Letters, Vol. 12, No.4, 1968, pp156-159.

Primary ExaminerLeon D. Rosdol Assistant Examiner-Harris A. PitlickAttorney, Agent, or FirmH. Frederick Hamann; G. Donald Weber, Jr.

[57] ABSTRACT A method utilizing vacuum evaporation techniques forgrowing monocrystalline films of lll-V compounds such as galliumarsenide on structurally dissimilar crystalline substrates. In apreferred embodiment, a single gallium arsenide source is heated to900-1000C at a subatmospheric pressure of about 10 to 10" torr toevaporate gallium and arsenic therefrom for recombination on a smooth,clean single crystal sapphire (or-alumina) substrate maintained at about580-595C.

10 Claims, 1 Drawing Figure I 7" I vAcu f m PUMPUM LU l2 PATENTED3.864.162

T0 VACUUM PUMP Ill METHOD OF FORMING GALLIUM ARSENIDE FILMS BY VACUUMEVAPORATION DEPOSITION BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to methods for forming films of lll-Vcompounds on substrates. More particularly, this invention relates to aphysical vapor deposition process utilizing a gallium arsenide source toform an epitaxial monocrystalline film of gallium arsenide onsubstrates, typically comprising a metallic oxide material such assapphire, that are of a dissimilar crystal structure.

2. Description of the Prior Art Epitaxial techniques for the growth offilms of compounds of elements from Groups 111 and V of the PeriodicTable on crystalline substrates are becoming increasingly important.This is partially a result of the stringent physical and electricalrequirements imposed on materials by increasingly complicated devicetechnology. Epitaxial films of Ill-V compounds such as gallium arsenide,a compound having excellent potential for application in semiconductorand related technology, have been grown on crystalline substrates bychemical vapor deposition (CVD), by flash evaporation, and by sputteringtechniques. Vacuum evaporation techniques have also been of interest, inpart because of the potential of these techniques for achieving highfilm purity. Additionally, vacuum evaporation techniques have potentialsimplicity of operation in that extraneous chemical reactants,carriergases and so forth may not be required.

As is well known, the physical, thermal and electrical characteristicsof single crystal sapphire (oz-A1 make it an excellent substratematerial. Accordingly, it is desirable to utilize the potentialadvantages of vac uum evaporation techniques to grow epitaxialmonocrystalline films of Ill-V compounds such as gallium arsenide onsapphire substrates.

A vacuum evaporation technique for depositing gal lium arsenide filmsonto crystalline substrates is taught in U.S. Pat. No. 3,476,593 toWilliam I. Lehrer. This technique utilizes separate sources of galliumoxide and arsenic and maintains the substrate temperature within thepreferred range 550600C. The use of this technique to grow epitaxialmonocrystalline gallium arse nide is limited to substrate materialshaving a crystal lattice and lattice constants similar to those ofgallium arsenide, specifically, monocrystalline germanium and silicon.

The use of vacuum evaporation techniques to grow epitaxial galliumarsenide films on monocrystalline substrates has also been reported byJohn E. Davey and Titus Pankey, in the Journal of Applied Physics, Vol.

39, No. 4, pp. 1941-48, (March 1968). A modified three-temperature zonetechnique employing argon bombardment and post-anneal was used. This wasa relatively complicated technique that required separate sources ofgallium and arsenic as well as separate heaters and temperatures for thetwo sources and the substrate. The authors were unsuccessful in a briefattempt to use this technique to grow epitaxial monocrystalline galliumarsenide on alumina substrates that were prepared using standardpolishing and etching techniques.

A modification of the three-temperature technique in which Group 111 andGroup V molecules are supplied from a molecular beam directed at thesubstrate has been used by J. R. Arthur and J. J. LePore for the vacuumgrowth of gallium arsenide and gallium phosphide on substrates of thesame compounds. See the Journal of Vacuum Science Technology, Vol. 6, p.545, ff (1969). In addition, U.S. Pat. No. 3,615,931 to J. R. Arthurteaches the epitaxial growth of films of III-V compounds by directingmolecular beams containing the llI-V compounds at a substrate heated to450650C. However, while the molecular beam technique incorporates someof the features of vacuum evaporation, the use of molecular beamspresents complicated equipment and operational requirements that areabsent in vacuum evaporation. Also, although U.S. Pat. No. 3,615,931includes sapphire in a list of commercially available substratematerials that are suitable for use with the molecular beam technique,the patent teaches that the suitable substrate materials are thosehaving lattice constants closely related to the film material. Forgallium arsenide films, specific application of the technique waslimited to gallium arsenide substrates. Also, there is no teaching of asingle source of film material.

As maybe appreciated, it is desirable to realize the potentialadvantages of vacuum evaporation for simplicity of operation and filmpurity in growing epitaxial, monocrystalline Ill-V compoundsemiconductors such as gallium arsenide on insulative substrates,including substrates of structurally dissimilar compounds such assapphire.

SUMMARY OF THE INVENTION A method of depositing an epitaxial,monocrystalline film of gallium arsenide upon a monocrystallineinsulating substrate comprises the steps of: (l polishing a surface ofthe substrate to a smooth finish; (2) etching the surface to a smooth,clean finish; (3) heating the substrate to a temperature within theapproximate range 580-595C at subatmospheric pressure; and (4) heatinggallium arsenide to 900-1000C at subatmospheric pressure to evaporategallium and arsenic from the source and to recombine the gallium andarsenic as stoichiometric gallium arsenide upon the surface of theheated substrate. Typically, the subatmospheric pressure is within therange 10 to 10 torr.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a front elevational view,partly in section, of a vacuum evaporation system for growingmonocrystalline films of materials on monocrystalline substrates.

DETAILED DESCRIPTION Shown in FIG. 1 is one embodiment of a vacuumevaporation system, designated generally by the reference numeral 10,that may be used in practicing the method of the present invention. Thesystem 10 includes an enclosure 11, such as a glass bell jar, that issupported by a base 12 and can be evacuated in a known manner by avacuum pump (not shown). Mounted within the enclosure 11 is a holder 13for a crucible 14 containing a single source 16 of a Group Ill-Vcompound such as gallium arsenide. As used here, the term single source"indicates a compound containing both gallium and arsenic, in contrast toseparate sources of gallium and arsenic, and encompasses a plurality ofcrucibles of the single source compound. One or more substrates 17 maybe secured by clips 18 to a holder 19 that is mounted proximate to andabove the crucible 14 and source 16 by standards 20. A substratethermocouple system 21 is is used in conjunction with a suitable powersupply (not shown) for controlling the operation of a substrate heater22 to control The mechanism of deposition in the present invention isthe evaporation of Ga and As; from the single heated source 16 and therecombination thereof as GaAs at the substrate surface 24. At atypicalsource 16 the temperature of the substrates 17. Likewise, asuittemperature of about 1000C, the vapor pressure ratio able powersupply and thermocouple (not shown) and of the Ga and A5 P /P isapproximately 60. Cona heater 23 are used to control the temperature ofthe sequently, Ga atoms and an excess of A5 molecules are crucible l4(and the source 16). supplied to the substrate. However, the presentinven- To practice the method of the present invention. tion utilizesthe fact thatthe reaction kinetics ofGaand monocrystalline substrates 17are l cut to the desired A52 are controfled at the substrate to attainstoichiocrystallographic orientation and poiished; etehed; metric GaAsfilms, as described subsequently. This is a brought to the desiredtemperature at subatmo' accomplished using the evaporation of GaAs froma Spheric pressure within the Vacuum evaporation System single sourcewithout the introduction of extraneous l0; n n the crucible l4 andSource 16 are chemical reactants, carrier gases and so forth. heated atsubatmospheric pressure to a temperature AS is known the an theCondensation coefficient and for a time sufficient to deposit a desiredthickness f Ga is approximately unity and the Surface cohcemra ofgaiiiiim arsenide the Substratestion thereof is temperature dependentabove about In carrying out the first step of the present invention,477C In contrast, A52 molecules normally haVe a very each P Singleerystai substrate 17 is cut so that a low condensation coefficient for aGaAs surface. How- Surtaoe 24 thereof eioseiy approximates theerystaiio' 2O ever, if absorbed Ga is present, the As condensation g ppiahe chose" for depositiohr For morioerystai' coefficient isproportional to the amount of Ga. Then, iihe iiirh growththe Surface 24is preferabiy within one if Ga and an excess of As; are supplied to thesubstrate, degree oi the depositioh piahe' as by evaporation from GaAs,a substantial percentage Continuing with step one, because the substratesurf the number f G atoms and a corresponding face smoothness isimportant in achieving hetero 25 ber of As molecules are retained at thesubstrate surepitaxiai mohoerystaiiihe growth the Substrate 17 is face,while the excess As is reflected. That is, stoichipolished sufficientlyto remove substantially all surface Ometry i attained roughnessincluding polishing Scratches- Preferabiyt The relative effect ofvarious parameters on the sucthe polished substrate is then washed in asuitable solcess of the instant vacuum evaporation method is i removegrease and residue deposited during the certain. However, it is believedthat the effect of each D of the various preparation and depositionsteps is im- Accordmg to step and immediately prior f Step portant toachieving deposition of monocrystalline three, the substrate 17 isetched to strip contaminants GaAs Thus the Substrate polishing andetching tech and polishing residue from the surface After etch niques,the substrate deposition temperature (particuing, the substrate 17 isrinsed in distilled water, washed lath, in terms of the reactionkinetics created by the in boiling methyl alcohol and dried substratetemperature), and the deposition rate are In carrying out step three,the substrate 17 IS secured deemed Significant to the substrate holder19 with the deposition surface 24 facing the crucible 14. Typically,polycrystalline gallium arsenide chunks that have been etched to remove40 EXAMPLES any surface impurities are placed in the crucible 14 to TheTable infra Summarizes the terrriperatui'eS p- Serve as h Source 16 Th hvacuum pump plied to various samples and the results obtained usingshown) is activated to evacuate the enclosure 11 to a the method of thepresent ihveritiohr Aithoiigh the desired equilibrium base pressure andthe substrate method is described speeiiieaiiy for the heteroepitaxiaiheater 22 is activated to bring the substrate 17 to equigrowth ofgallium arsenide films on sapphire subst ate librium at the desiredoperating temperature. it is app in general to Ill-V or p un Step fouris initiated by actuating the crucible heater mS- Additionally, thesubstrates may comprise other 23 to elevate the temperature of thecrucible l4 and materials, iheiudihg metaiiio oxides Such as Spiheisource 16 to a range suitable for evaporation of gallium g 204) ndberyllia 6 Which are r r lly and arsenic. Step four is normallyinitiated concureither similar or dissimilar to the filmrently with, orsubsequent to, the heating of the sub- As mentioned above, the method ofthe present instrate 17 because earlier heating of the source mightvention is suited for growing films on substrates of diftend toprematurely deplete the arsenic. Although the ferent crystal structure.As used here, the term crystal preferred gallium arsenide sourcetemperature is 900 structure is defined to include crystal lattices andlatto i000C, higher or lower temperatures may be utitice constants. Theexamples specifically concern films lized with satisfactory results. Ashutter 26 is interor layers ofgallium arsenide (face centeredcubic,zincposed between the crucible 14 and the substrate 17 blendearrangement; a 5.65 A.) on sapphire (themuntil thermal equilibrium isattained, thereby precludbohedral symmetry, distorted hexagonal packingwith ing premature deposition on the substrate. a '=4.76A. c 13.00 A.).

TABLE SUBSTRATE SAMPLE NO. TEMPERATURE,C STRUCTURAL CHARACTERISTICS OFGROWN GaAs FILM A C C l 200 Amorphous 2 300 Polycrystalline 3 H2 do. 4480 Polycrystalline 4 4X0 Polycrystalline TABLE Continued SUBSTRATESAMPLE NO. TEMPERATURE.C STRUCTURAL CHARACTERISTICS OF GROWN GaAs FILM"A C C 5 484 do. 6 486 do. 7 50] Polycrystalline with monocrystallinity 8546 Monocrystalline with trace of polycrystallinity 8 546Polycrystalline 9 552 Monocrystalline with trace of preferredorientation l 84 Monocrystalline l l 589 do.

I l 589 do. 12 600 Monocrystalline with trace of pol ycrystallinity I Aindicates in-house polished substrates Verneuil-grown, monocrystallinesapphire substrates 17 were cut to about 0.015 inches thickness, with{0001} planes within one degree of the deposition surfaces 24. Surfaceroughness and imperfections may be critical to any failure to achievemonocrystalline gallium arsenide films. To avoid such failure, and tomore precisely establish the effect of temperature on epitaxial growth,at least one substrate having a smooth, A polish deposition surface 24was used for each substrate temperature investigated. The A polishingsequence comprised polishing the deposition surface 24 with successivelyfiner diamond paste to 1.0 micron and then finish polishing with 0.3micron Linde A alumina.

After polishing, the substrates 17 were degreased in trichloroethylene.Then, according to step two, the substrates were etched in an etchantsolution comprising a 2:9 mixture of HFzHNO to remove surfaceimpurities. As mentioned previously, the substrates were then rinsed indistilled water, washed in boiling methyl alcohol, and air dried.

According to a substep of step two, surface impurities were removed fromthe polycrystalline gallium arsenide chunks that were used as the source16. As an example, the chunks were prepared by etching for about oneminute in a solution of methanol plus one C" indicates commerciallypolished substrates percent bromine, then rinsing in boiling methanoland air drying.

ln carrying out step three, the polished and etched sapphire substratesl7 and the etched gallium arsenide single source 16 were positioned,respectively, on the holder 19 and within the crucible 14. The vacuumpump (not shown) was then activated to evacuate the enclosure 11 to anequilibrium base presssure of approximately 10 to 10"torr. As theenclosure 11 approached equilibrium subatmospheric pressure, thesubstrate heater 22 was activated to elevate the substrates l7 and, moreimportantly, their surfaces 24 to a temperature within the investigativerange of 200600C. Using the simple thermocouplecontrolled heater 22 inFIG. 1, the substrate temperatures were easily maintained at within adegree of the desired equilibrium temperature.

The crucible heater 23 used for step four was a tantalum shieldedtungsten wire basket which maintained the source 16 within a suitableevaporation range of 900-l000C. Using a substrate 17 temperature ofabout 590C, 21 substrate-to-source distance of about two centimeters,and a source 16 temperature of about l000C, the vacuum evaporationsystem 10 achieved GaAs deposition rates of about 0.1 to 0.15 micronsper minute.

The structure of the GaAs films was evaluated using reflection electrondefraction (RED) at kv. with the electron beam at an angle of one degreerelative to the film surfaces. The RED results indicated the growncrystalline films were all pure, stoichiometric gallium arsenide.

The structures of four films grown on commercially polished substrates,hereinafter termed C films, were evaluated using RED. The C films, whichwere grown for the substrate temperature range of 4805 89C, are listedin the Table as sample No.s. 4C, 5C, 8C and 11C corresponding totemperatures of 480, 484, 546, and 589C. The C films were primarilypolycrystalline with some preferred orientation.

ln contrast to the C films, most of the finely polished A filmsexhibited a tendency toward increased quality, i.e., monocrystallinity,for increasing temperatures to about 600C, with {111} planes growingparallel to the (0001) deposition plane of the substrate. Film samplenumber lA, grown at a substrate equilibrium temperature of 200C, wasamorphous. For the range 300-486C, the gallium arsenide films 2A-6A werepolycrystalline. Film samples 7A, 8A and 9A for 50lC, 546C and 552Cwere, respectively, predominately polycrystalline, predominatelymonocrystalline, and almost entirely monocrystalline. At 584C and 589C,the film samples 10A and HA were entirely monocrystalline.

Sample 12A (600C) was similar to sample 8A (546C) in that it wassubstantially monocrystalline with traces of polycrystallinity.

Based upon the above-described characteristics of the grown films, it isconservatively estimated that monocrystalline gallium arsenide filmswere achieved for the substrate temperature range of about 580-595.

RED indicated twinning in certain lll directions. However, the twindensity is lowered considerably by the method of the present inventionand is sufficiently low to preclude deleterous effects on thefunctioning of devices fabricated from the gallium arsenide on sapphiresamples. In addition, twin densities would decrease with furtherrefinements which are within the scope ofthe present invention. Forexample, the nucleation process that initiates film growth on thesubstrate could be at a given temperature, with subsequent growth at ahigher (or lower) temperature. Alternatively, the deposition rate couldbe varied during growth. Other alternatives include post growthannealing, thermal cycling, and combinations of the above.

The quality of the gallium arsenide films was also checked using Laueback reflection X-ray diffraction and unfiltered copper radiation. Theresults were considered to be in agreement with the RED results,although the indicated film quality was not as consistently good attemperatures other than 584 and 589C. This difference in results is notunexpected however. This is because films are frequently of betterquality near the surface and the RED findings are indicative of thequality within several hundred angstroms of the surface of the film,while X-ray diffraction represents a sampling of the entire film body.It would thus seem that the gallium arsenide films exhibit thecharacteristic of enhanced crystallographic quality at the surface.

Thus, there has been described a method of forming a layer ofmonocrystalline Ill-V compound on a substrate having a crystal structuredissimilar to the crystal structure of the layer. Preferred compounds,temperatures and the like have been described. Alternative compounds andparameters have been indicated. The scope of the invention is limited,however, only by the claims appended hereto and equivalents thereto.

Having thus described a preferred embodiment of the invention, what isclaimed is:

l. A method of forming a layer of monocrystalline gallium arsenide on amonocrystalline substrate having a dissimilar crystal structure,comprising:

removing a sufficient thickness of substrate mateial from at least onesurface of the substrate to define a substantially smooth surface;

etching said surface to define a clean surface finish;

heating a single source of gallium arsenide at a subatmospheric pressureof approximately 10 to torr to evaporate gallium and arsenic; and

heating the substrate at the subatmospheric pressure to a temperaturesufficient to condense stoichiometric gallium arsenide on said onesurface of said substrate.

2. A method as defined in claim 1, wherein the substrate temperature iswithin the approximate range SSW-600C.

3. A method as defined in claim 1, wherein the substrate temperature iswithin the approximate range 580-595C.

4. A method as defined in claim 1 wherein the substrate is sapphire.

5. A method as defined in claim 1 wherein the substrate is MgAl O 6. Amethod as defined in claim 1 wherein the substrate is BeO.

7. A method of forming a layer of monocrystalline gallium arsenide on amonocrystalline sapphire substrate, comprising:

polishing at least a surface of the substrate;

etching the polished surface to a smooth, clean finish;

heating the etched surface of the substrate to a temperature ofapproximately 580595C at a subatmospheric pressure of approximately 10to l0 torr.; and

heating a single source of gallium arsenide at the subatmosphericpressure range and in the presence of the heated substrate to evaporategallium and arsenic from the single source for recombination on thesubstrate surface.

8. A method of forming a layer of monocrystalline gallium arsenide on amonocrystalline sapphire substrate as defined in claim 7, wherein thesingle source of gallium arsenide is maintained within the temperaturerange 9001000C.

9. A method of epitaxially forming a layer of monocrystalline galliumarsenide as defined in claim 7,

wherein the deposition surface of the substrate is a.

000i} plane and gallium arsenide is formed with a plane of the type l l1} parallel to the deposition plane.

10. A method of forming a layer of monocrystalline gallium arsenide on amonocrystalline sapphire substrate as defined in claim 7, wherein thepolishing sequence comprises polishing said surface with successivelyfiner diamond paste to approximately 1.0 mi cron, then finish polishingwith approximately 0.3 micron alumina, and wherein the etching steputilizes an etchant solution comprising an approximately 2:9 mixture ofHF:HNO

2 g UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent3.864.162 Dated February 4, 1975 Inventor(s) Joseph L. Kenty It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Item [257 after "Fi1ed:", change "Feb. 4, 1975" to -July 16, l973--.

anti sealed thlS lStn cay 05 91 1 1. 1, 1

1. A METHOD OF FORMING A LAYER OF MONOCRYSTALLINE GALLIUM ARSENIDE ON AMONOCRYSTALLINE SUBSTRATE HAVING A DISSIMILAR CRYSTAL STRUCTURE,COMPRISING: REMOVING A SUFFICIENT THICKNESS OF SUBSTRATE MATERIAL FROMAT LEAST ONE SURFACE OF THE SUBSTRATE TO DEFINE A SUBSTANTIALLY SMOOTHSURFACE; ETCHING SAID SURFACE TO DEFINE A CLEAN SURFACE FINISH; HEATINGA SINGLE SOURCE OF GALLIUM ARSENIDE AT A SUBATMOSPHERIC PRESSURE OFAPPROXIMATELY 10**-5 TO 10**-8 TORR TO EVAPORATE GALLIUM AND ARSENIC;AND HEATING THE SUBSTRATE AT THE SUBATMOSPHERIC PRESSURE TO ATEMPERATURE SUFFICIENT TO CONDENSE STOICHIOMETRIC GALLIUM ARSENIDE ONSAID ONE SURFACE OF SAID SUBSTRATE.
 2. A method as defined in claim 1,wherein the substrate temperature is within the approximate range550*-600*C.
 3. A method as defined in claim 1, wherein the substratetemperature is within the approximate range 580*-595*C.
 4. A method asdefined in claim 1 wherein the substrate is sapphire.
 5. A method asdefined in claim 1 wherein the substrate is MgAl2O4.
 6. A method asdefined in claim 1 wherein the substrate is BeO.
 7. A method of forminga layer of monocrystalline gallium arsenide on a monocrystallinesapphire substrate, comprising: polishing at least a surface of thesubstrate; etching the polished surface to a smooth, clean finish;heating the etched surface of the substrate to a temperature ofapproximately 580*-595*C at a subatmospheric pressure of approximately10 5 to 10 8 torr.; and heating a single source of gallium arsenide atthe suBatmospheric pressure range and in the presence of the heatedsubstrate to evaporate gallium and arsenic from the single source forrecombination on the substrate surface.
 8. A method of forming a layerof monocrystalline gallium arsenide on a monocrystalline sapphiresubstrate as defined in claim 7, wherein the single source of galliumarsenide is maintained within the temperature range 900*-1000*C.
 9. Amethod of epitaxially forming a layer of monocrystalline galliumarsenide as defined in claim 7, wherein the deposition surface of thesubstrate is a (0001) plane and gallium arsenide is formed with a planeof the type (111) parallel to the deposition plane.
 10. A method offorming a layer of monocrystalline gallium arsenide on a monocrystallinesapphire substrate as defined in claim 7, wherein the polishing sequencecomprises polishing said surface with successively finer diamond pasteto approximately 1.0 micron, then finish polishing with approximately0.3 micron alumina, and wherein the etching step utilizes an etchantsolution comprising an approximately 2:9 mixture of HF:HNO3.